I am thinking of modifying an existing IDE (Ex : By developing a plugin) to provide support for a proprietary scripting language. I just need to implement few features like syntax highlighting, Autocompleting etc. (i.e the requirements are really simple). What would be the best IDE or Text editor to integrate the feature. As an example if I think to develop an eclipse plugin for that it would be a pain.
What do you think about Notepad ++?
In vim you can easily add your custom syntax highlighting rules by adding another file in the syntax folder; for the details it is best to look at the help.
If I remember correctly notepad++ also allows defining custom syntax files.
You might want to give the NetBeans Generic Language Framework a try.
NetBeans project called Generic Languages Framework allows you to define a programming language and integrate it to NetBeans IDE. In the first step you should describe the language - lexical part (define tokens using regular expressions) and syntax (grammar rules). In the second step you can define how to visualize this language in NetBeans. You can define colors for tokens (syntax coloring) or folding (based on grammar rules). You can define what parts of this language should be displayed in the navigator, and how to indent this language plus many other features.
This tutorial should guide you through the process of creating new NetBeans module, adding languages support into it, describing lexical and syntax structure of your language and adding support for all standard editor # features for your language.
Notepad++ allows you to define custom syntax highlight files in a very easy way, but it's not a very good solution for auto-completion (look at this SO question).
If you want a real IDE to extend, I suggest you to use Eclipse.
Update: Tutorial on how to develop an Eclipse plugin.